QUESTIONS & ANSWERS
what the transistor on the right is for,
or how to connect the two switches.
The DC adapter is probably good.
Your guess is as good as mine on the
lamp specs. If you get to the point of
hooking it up, if it lights and doesn’t
burn out it is good. A reader may
have the instruction manual; if so, I
will let you know.
POWER SUPPLY/JUN ’ 12
QI follow your column every month with great interest. If my reasoning is correct, the reason for the C1-C3
capacitors is to introduce a phase
angle which will lower the power
dissipation that would result without
them. What I don’t understand is how
you determine the ratio of XL:R1.
— Mel Menders
■ FIGURE 6.
AThanks for asking. The capacitor is dropping most of the voltage, so it is the dominant phasor. I ignore
the phase angle and use:
I = 120/(Xl + R)
To be more accurate, the
equation should be:
I = 120/(Xl2 + R2)1/2.
I also ignore the effect of
rectification because it is just too
Dear Russell: Re: Modifications to a Simple Radio,
June ‘ 12, page 24, Figure 2.
I have four questions since I am not an RF guy:
• C1: How long can the antenna be?
— Toby Norton
Response: The longer the antenna the better, in most
cases. I used to use a 50 foot antenna from my bedroom
window to the peak of the barn with good results.
Yes, C2 is a variable capacitor.
L3 and L4 can be on the same form; in fact, that is true
of L2 also.
C4 can be connected to any audio amp; the 386 would
Dear Russell: Re: Transformerless Power Supply, June
‘ 12, page 26. Cute solution to the problem posed. However, I
am troubled by the "no load" condition in the subject circuit.
Your two-diode solution presumes that the two diodes are
identical in zener voltage, never mind individual thermal
effects. The 1N4744A is a 5% tolerance device, meaning that
there could be nearly 10% voltage difference between two
given devices, whence one would take up all of the load
(until it overheated). Better you should have used two
1N4738A diodes connected in series, with each dissipating
860 mW (worst case at 100 mA). The overall voltage across
the pair will not exceed the range 14. 8-18.0 volts. Then, use a
7812 post regulator (TO-220 package) which can stand the
688 mW dissipation at 18 volts input and 11. 4 volts output.
Otherwise, use a zener follower circuit to pre-regulate to
~ 15 volts.
— Peter A. Goodwin
Response: You are right; thanks for writing. I wish I had
done it that way.
Dear Russell: Re: Multi Input A/V Switch, June ‘ 12. First
of all, I want to say I enjoy your column; in all honesty it's
the second thing I read after the Tech Forum. I have been in
the electronics field — two-way radio in particular — and
feel that no matter what I know there are still things I can
learn from others.
With that being said, I have a problem with your answer
to Derek Trombrello. He wrote saying that he had 25 video
games to switch (sounds like someone with too much time);
however, in his query he stated there was composite video
and stereo to be switched; he mentioned the normal color
coding: yellow-video, red-right audio, and white-left audio;
although the article states "The cables are composite A/V
(red, white, yellow, and stereo audio)."
The thing that got my attention was five lines that had
to be switched. I am at a loss as to why there would be five
I/Os unless the video games were putting out HD. If it is
composite video, shouldn't there only be three I/Os to be
switched; two for audio and one for video?
— Craig Kielfofer
Response: Your analysis is correct, thanks for writing. I
was confused as to the number of lines to be switched, but
when I sent the schematic to Derek Trombrello, he replied
telling me that there were only three lines, video, and stereo
audio. By that time, I had already submitted the column so I
let it go, reasoning that an excess of lines was no problem.
September 2012 23